1. Field of the Invention
The present invention relates to a method for manufacturing high-temperature thermistor materials having widely selectable values of resistivity and temperature coefficients of resistance, and also relates to high-temperature thermistors using the materials.
2. Description of the Related Art
High temperature thermistors are temperature sensors that are employed to measure the flame temperature in a hot water supply, the temperature of a heating furnace or the exhaust gas temperature of a motor vehicle at temperature from about 100.degree. C. to about 1300.degree. C.
In the prior art (Japanese Unexamined Patent Application No. 5-62805), it is shown that thermistor materials for constituting high-temperature thermistor elements are made from a sintered body that is mixed and fired (Mn.Cr) O.sub.4 and YCrO.sub.3, so that the values of resistivity and temperature coefficient of resistance are selectable over a wide range.
(Mn.Cr) O.sub.4 is a high-temperature thermistor material and has a high resistivity and a high temperature coefficient of resistance. On the other hand, YCrO.sub.3 is a similar thermistor material and has a low resistivity and a low temperature coefficient of resistance. Therefore, the high-temperature thermistor can be given a desirable resistivity and a suitable temperature coefficient of resistance by suitably varying the mixing ratio of the (Mn.Cr) O.sub.4 and YCrO.sub.3. The high-temperature thermistor material has a wide range of stable thermistor properties.
However, there are some problems, as follows, in the prior art thermistor materials. Advantageous properties of a thermistor material become possible by uniformly dispersing (Mn.Cr) O.sub.4 particles and YCrO.sub.3 particles in the material. As mentioned above, because both (Mn.Cr) O.sub.4 having better resistivity and better temperature coefficient of resistance and YCrO.sub.3 having worse resistivity and worse temperature coefficient of resistance are uniformly mixed and dispersed together throughout the thermistor material, the resistivity and the temperature coefficient of resistance of the thermistor material are homogenized throughout the thermistor material.
Thus, since the bonding force between the particles is not too large, the particles in the above material are liable to gradually separate over time. Because this separated thermistor material may bring about changes in the thermistor properties, the thermistor can not detect an exact temperature.
Therefore, it is not easy to obtain stable thermistor properties from the high-temperature thermistors made of the above prior art material.